This invention relates generally to novel multilayer synthetic-resin laminated structures and a process for producing the same. More particularly, the invention relates to a process for producing a multilayer composite formed article from an olefin-containing polymer and a polymer containing nitrile, which process is characterised by the interposition of a polymer mixture formed by blending an olefin-containing polymer and a nitrile-containing polymer between an olefin-containing polymer layer and a nitrile-containing polymer layer for the purpose of imparting high adhesive strength between the two polymer layers.
Heretofore, olefin-containing polymers have been widely used in many fields as packaging materials in formed articles such as blow-molded bottles, sheet, films, and tubes. Olefin-containing polymers, however, have the characteristic of having high permeability with respect to inorganic gases such as oxygen and carbon dioxide gases and to organic gases such as hydrocarbon gases. For this reason, these materials have not yet been used in applications requiring a gas-barrier characteristic as in the fields of packaging of products such as foodstuffs, pharmaceutical products, and cosmetics.
On one hand, nitrile-containing polymers, in general, have very low gas permeability with respect to inorganic and organic gases, but their gas-barrier characteristic with respect to water vapor cannot be said to be satisfactory in comparison with that of olefin-containing polymers. Furthermore, olefin-containing polymers, in general, possess excellent pliability and also high impact strength and also have good resistance to low temperatures. On the other hand, polymers containing nitrile, in general, have high rigidity and poor pliability in the case of products with a thick wall, and insufficient impact strengths. Accordingly, with the aim of making most of the desirable characteristics of the two kinds polymers, a method wherein the two kinds of polymers are rendered into a composite structure by superimposing thereof in layered or laminated form has been used. By this method, materials possessing excellent properties are produced.
In general, resins of various kinds are rendered into a composite material of layered form by mutual adhesion by a method which comprises prefabricating beforehand the materials in layered form and then applying heat and pressure thereto to cause adhesion by heat fusion or by a melt adhesion method which conprises cocurrent melt extrusion (coextrusion) by which the individual resins are caused in molten state to adhere to each other within or outside of the extrusion die. Still another method is the so-called lamination method wherein layer materials formed beforehand are bonded together with an adhesive.
In the production of a composite formed product by bonding together a nitrile-containing polymer and an olefin-containing polymer, however, these resins cannot be amply bonded together by any of these methods, or, even if bonding thereof is possible, various restrictions are imposed on the materials or the fabrication process, whereby disadvantages in production or economy cannot be avoided. More specifically, in the case of forming a composite structure by the lamination method, various restrictions relating to properties and performance are imposed on the adhesive, and only a limited number of kinds thereof can be used. Furthermore, the adhesive itself is expensive, and, moreover, the application of the adhesive is complicated, making this technique disadvantageous in production and cost.
The method of bonding the resins in molten state, in general, is simple, convenient and economical. However, when an attempt is made to bond by the heat fusion method of a nitrile-containing polymer (I) and an olefin-containing polymer (III), sufficient adhesion between the two polymers cannot be obtained, and the melt adhering method such as coextrusion cannot be effected.